Estrogens are well-known endocrine regulators in the cellular processes involved in the development and maintenance of the reproductive system. Estrogens have also been shown to have important effects in many non-reproductive tissues such as bone, liver, the cardiovascular system, and the central nervous system. The most widely accepted hypothesis of how estrogens exert their effects is by binding to an intracellular steroid hormone receptor. After the receptor and bound ligand are transferred to the nucleus of the cell, the complex binds to recognition sites in DNA, which allows for the modulation of certain genes. Additionally, it is now becoming apparent that estrogens may mediate their effects via membrane-initiated signaling cascade, though much of this work is still experimental. Kousteni et al., Journal of Clinical Investigation, (2003), 111, 1651-1664, herein incorporated by reference with regard to such teaching.
Certain substances have demonstrated the ability to exhibit their biological activity in a “tissue-selective” manner. In other words, tissue selectivity allows functionality as estrogen agonists in certain tissues, while acting as estrogen antagonists in other tissues. The term “selective estrogen receptor modulators” (SERMs) has been given to these molecules. Examples of SERMs include tamoxifen, raloxifene, lasofoxifene, clomiphene, and nafoxidine. The molecular basis for this tissue-selective activity is not completely understood. Without being limited to any particular theory, the ability of the ligand to place the estrogen receptor into different conformational states and allowing for differential capabilities in recruiting coactivator and corepressor proteins, as well as other important proteins involved in transcriptional regulation, is believed to play a role. See, McDonnell, D. P., The Molecular Pharmacology of SERMs, Trends Endocrinol. Metab. 1999, 301-311, herein incorporated by reference with regard to such description.
Historically estrogens were believed to manifest their biological activity through a single estrogen receptor, now termed estrogen receptor alpha (ERα). More recently, however, there was the discovery of second subtype of estrogen receptor, termed estrogen receptor beta (ERβ). See, Kuiper et al., WO 97/09348 and Kuiper et al., Cloning of a Novel Estrogen Receptor Expressed in Rat Prostate and Ovary, Proc. Natl. Acad. Sci. U.S.A., 1996, pp. 5925-5930, each herein incorporated by reference with regard to such subtype. ERβ is expressed in humans. See, Mosselman et al., ERβ: Identification and Characterization of a Novel Human Estrogen Receptor, FEBS Lett., 1996, pp. 49-53, herein incorporated by reference with regard to such expression. The discovery of this second subtype of estrogen receptor significantly increased the biological complexity of estrogen signaling and may be responsible for some of the tissue-selective actions of the currently available SERMs.
As noted above, estrogens have important effects in many non-reproductive tissues. Thus, estrogen modulation is believed useful in the treatment or prophylaxis of diseases and conditions associated with such tissues, including bone, liver, and the central nervous system.
For example, osteoporosis is characterized by the net loss of bone mass per unit volume. Such bone loss results in a failure of the skeleton to provide adequate structural support for the body, thereby creating an increased risk of fracture. One of the most common types of osteoporosis is postmenopausal osteoporosis, which is associated with accelerated bone loss subsequent to cessation of menses and declining levels of endogenous estrogen in women. There is an inverse relationship between densitometric measures of bone mass and fracture risk, for peri- and postmenopausal women in the process of rapid bone loss due to declining levels of estrogen. See, Slemenda, et al., Predictors of Bone Mass in Perimenopausal Women, A Prospective Study of Clinical Data Using Photon Absorptiometry, Ann. Intern. Med., 1990, pp. 96-101 and Marshall, et al., Meta-Analysis of How Well Measures of Bone Mineral Density Predict Occurrence of Osteoporotic Fractures, Br. Med. J., 1996, pp. 1254-1259, each of which is herein incorporated by reference with regard to such relationship. Elderly women currently have a lifetime risk of fractures of about 75%. In addition there is an approximate 40% risk of hip fracture for Caucasian women over age 50 in the United States. The economic burden from osteoporotic fractures is considerable because of the necessity of hospitalization. In addition, although osteoporosis is generally not thought of as life-threatening, the mortality within 4 months of hip fracture is currently approximately 20 to 30%. Current therapies for postmenopausal osteoporosis include hormone replacement therapy or treatment with other antiresorptive agents such as bisphosphonates or calcitonin. Similarly, SERMS have been shown to be effective in the treatment of postmenopausal osteoporosis (see, Lindsay, R.: Sex steroids in the pathogenesis and prevention of osteoporosis. In: Osteoporosis 1988. Etiology, Diagnosis and Management. Riggs B L (ed)I, Raven Press, New York, USA (1988):333-358; Barzel US: Estrogens in the prevention and treatment of postmenopausal osteoporosis: a review. Am J. Med (1988) 85:847-850; and Ettinger, B., Black, D. M., et al., Reduction of Vertebral Fracture Risk in Postmenopausal Women with Osteoporosis Treated with Raloxifene, JAMA, 1999, 282, 637-645, each of which is incorporated by reference with regard to such teaching).
As another example, the effects of estrogens on breast tissue, particularly breast cancer, have been well documented. For example, a previously identified SERM, tamoxifen, decreases the risk of recurrent breast cancer, contralateral breast cancer, and mortality as well as increases the disease-free survival rate of patients with breast cancer at multiple stages of the disease. See, Cosman, F., Lindsay, R. Selective Estrogen Receptor Modulators: Clinical Spectrum, Endocrine Rev., 1999, pp. 418-434, herein incorporated by reference with regard to such teaching. The profile of tamoxifen, however, is not ideal due to potential interactive properties on reproductive tissues, such as uterine tissues. There is room for an improved therapy for the treatment of such cancers, namely a SERM with no agonist properties on any reproductive tissues.
Cardiovascular disease is the leading cause of death among postmenopausal women. Until recently, the preponderance of data suggested that estrogen replacement therapy in postmenopausal women reduced the risk of cardiovascular disease, although some studies reported no beneficial effect on overall mortality. See, Barrett-Connor, E. et al., The Potential of SERMs for Reducing the Risk of Coronary Heart Disease, Trends Endocrinol. Metab., 1999, pp. 320-325, herein incorporated by reference. The mechanism(s) by which estrogens were believed to exert their beneficial effects on the cardiovascular system are not entirely clear. Potentially estrogen's effects on serum cholesterol and lipoproteins, antioxidant properties, vascular smooth muscle proliferation, and inhibition of arterial cholesterol accumulation were believed to play a role. Id. See also, Cosman, F., Lindsay, R. Selective Estrogen Receptor Modulators: Clinical Spectrum, Endocrine Rev., 1999, pp. 418-434, herein incorporated by reference. In light of the recent reports of the HERS II and WHI studies, however, continuous combined Hormone Therapy, namely, CEE+MPA [Conjugated Equine Estrogen+Medroxy Progesterone Acetate], confers no cardiovascular benefit in menopausal women. See, Hulley S., Grady, D., Bush, T., et al., Randomized trial of estrogen plus progestin for secondary prevention of coronary heart disease in postmenopausal women. Heart and Estrogen/progestin Replacement Study (HERS) Research Group. J. Am. Med. Assoc. (1998) 280:605-613 and Wassertheil-Smoller S., Hendrix, S. L., Limacher, M., et al., for the WHI Investigators. Effect of estrogen plus progestin on stroke in postmenopausal women: the Women's Health Initiative: a randomized trial. JAMA (2003) 289, 2673-2684, each herein incorporated by reference with regard to such teaching. To what extent these findings may be extrapolated to SERMs is an issue that remains to be determined.
Other therapeutic alternatives include estrogen replacement therapy and/or hormone replacement therapy (“HRT”), which may be useful in the treatment of vasomotor symptoms, genitourinary atrophy, depression, and diabetes. Over 75% of women experience vasomotor symptoms during the climacteric years. Clinical signs, such as vasomotor symptoms and genitourinary atrophy, abate upon treatment with estrogen replacement therapy. Sagraves, R., J. Clin. Pharmacol. (1995), 35(9 Suppl):2S-10S, herein incorporated by reference with regard to such teaching. Preliminary data suggest that estradiol may alleviate depression during perimenopause and that the combination of estrogens and selective serotonin reuptake inhibitors may alleviate depression during the postmenopausal period. Soares, C. N., Poitras, J. R., and Prouty, J., Drugs Aging, (2003), 20(2), 85-100, herein incorporated by reference with regard to such teaching. Furthermore, hormone replacement therapy may improve glycemic control among women with diabetes. Palin, S. L. et al., Diabetes Research and Clinical Practice, (2001), 54, 67-77; Ferrara, A. et al., Diabetes Care, (2001), 24(7), 1144-1150), each incorporated herein by reference with regard to such teaching. There is a need, however, for improved therapies that present better side effect profiles as compared to HRT.
The present inventors discovered a novel group of prodrugs that deliver a parent compound, or compound 1. The parent compound binds to and modulates estrogen receptor alpha (“ER-α”) and estrogen receptor beta (“ER-β”). Thus, as a SERM, this compound is believed to be useful for the treatment and/or prophylaxis of, without limitation, menopausal or postmenopausal disorders, vasomotor symptoms, urogenital or vulvar vaginal atrophy, atrophic vaginitis, female sexual dysfunction, breast cancer, depressive symptoms, diabetes, bone demineralization, and the treatment and/or prevention of osteoporosis.
This parent compound 1 has poor bioavailability and, as such, is not viable for use as a pharmaceutical agent. The parent compound cannot be administered effectively by an oral route of administration because of poor systemic absorption. There exists a need, therefore, for one or more prodrug forms of compound 1 that can provide an appropriate profile for a commercial pharmaceutical agent, including but not limited to bioavailability, metabolism, stability, synthesis, and large-scale manufacture.